Field of the Invention
Implementations of the present invention relate generally to components and systems for drilling. In particular, implementations of the present invention relate to drill components comprising increased strength and resistance to jamming, cross-threading and wedging.
Relevant Technology
Threaded connections have been well known for ages, and threads provide a significant advantage in that a helical structure of the thread can convert a rotational movement and force into a linear movement and force. Threads exist on many types of elements, and can be used in limitless applications and industries. For instance, threads are essential to screws, bolts, and other types of mechanical fasteners that may engage a surface (e.g., in the case of a screw) or be used in connection with a nut (e.g., in the case of a bolt) to hold multiple elements together, apply a force to an element, or for any other suitable purpose. Threading is also common in virtually any industry in which elements are mechanically fastened together. For instance, in plumbing applications, pipes are used to deliver liquids or gasses under pressure. Pipes may have threaded ends that mate with corresponding threads of an adjoining pipe, plug, adaptor, connector, or other structure. The threads can be used in creating a fluid-tight seal to guard against fluid leakage at the connection site.
Oilfield, exploration, and other drilling technologies also make extensive use of threading. For instance, when a well is dug, casing elements may be placed inside the well. The casings generally have a fixed length and multiple casings are secured to each other in order to produce a casing of the desired height. The casings can be connected together using threading on opposing ends thereof. Similarly, as drilling elements are used to create a well or to place objects inside a well, a drill rod or other similar device may be used. Where the depth of the well is sufficiently large, multiple drill rods may be connected together, which can be facilitated using mating threads on opposing ends of the drill rod. Often, the drill rods and casings are very large and machinery applies large forces in order to thread the rods or casings together.
Significant efforts have been made to standardize equipment in oilfield, exploration and other drilling industries. In the case of drill rods, both outer and inner diameter standards have been developed and, in the case of threading, multiple threading standards have been developed to allow different manufacturers to produce interchangeable parts. For instance exemplary standardization schemes comprise Unified Thread Standard (UTS), British Standard Whitworth (BSW), British Standard Pipe Taper (BSPT), National Pipe Thread Tapered Thread (NPT), International Organization for Standardization (ISO) metric screw threads, American Petroleum Institute (API) threads, and numerous other thread standardization schemes.
While standardization has allowed greater predictability and interchangeability when components of different manufactures are matched together, standardization has also diminished the amount of innovation in drill component design. In one example, both outer and inner diameters of drill rods have been fixed by industry requirements. Accordingly, the portion of the wall thickness allocated to mating threads operable to transfer drilling loads and to withstand wear due to repeated making and breaking of the drill components must be balanced with the remaining material over the threaded portions of components so that the components can withstand drilling loads and wear due to abrasion against the drilled hole wall and resulting cuttings.
In another example, threads may be created using existing cross-sectional shapes—or thread form—and different combinations of thread lead, pitch, and number of starts. In particular, lead refers to the linear distance along an axis that is covered in a complete rotation. Pitch refers to the distance from the crest of one thread to the next, and start refers to the number of starts, or ridges, wrapped around the cylinder of the threaded fastener. A single-start connector is the most common, and comprises a single ridge wrapped around the fastener body. A double-start connector comprises two ridges wrapped around the fastener body. Threads-per-inch is also a thread specification element, but is directly related to the thread lead, pitch, and start.
While existing threads and thread forms are suitable for a number of applications, continued improvement is needed in other areas such as in high torque, high power, and/or high speed applications. In one instance, existing thread designs are inherently prone to jamming. In another instance, existing thread designs do not use available material effectively. In another embodiment, existing thread designs detract from load capacity of mated components. In yet another instance, existing thread designs exhibit excessive wear.
Jamming is the abnormal interaction between the start of a thread and a mating thread, such that in the course of a single turn, one thread partially passes under another, thereby becoming wedged therewith. Jamming can be particularly common where threaded connectors are tapered. In another instance, existing drill component designs can have limited drilling load capacity and fatigue load capacity as a result of the material afforded to the male thread or to the underlying material on the male end of a drill component.
In certain applications, such as in connection with drill rigs, multiple drill rods, casings, and the like can be made up. As more rods or casings are added, interference due to wedging or cross-threading can become greater. Indeed, with sufficient power (e.g., when made up using hydraulic power of a drill rig) a rod joint can be destroyed. Coring rods in drilling applications also often have threads that are coarse with wide, flat threaded crests parallel to mating crests due to a mating interference fit or slight clearance fit dictated by many drill rod joint designs. The combination of thread tails and flat, parallel thread crests on coarse tapered threads creates an even larger potential for cross-threading interaction, which may not otherwise be present in other applications.
In tapered threads, the opposing ends of male and female components may be different sizes. For instance, a male threaded component may taper and gradually increase in size as distance from the end increases. To accommodate for the increase in size, the female thread may be larger at the end. The difference in size of tapered threads also makes tapered threads particularly prone to jamming, which is also referred to as cross-threading. Cross-threading in tapered or other threads can result in significant damage to the threads and/or the components that include the threads. Damage to the threads may require replacement of the threaded component, result in a weakened connection, reduce the fluid-tight characteristics of a seal between components, or have other effects, or any combination of the foregoing.
For example, tail-type thread starts have crests with a joint taper. If the male and female components are moved together without rotation, the tail crests can wedge together. If rotated, the tail crests can also wedge when fed based on relative alignment of the tails. In particular, as a thread tail is typically about one-half the circumference in length, and since the thread has a joint taper, there is less than half of the circumference of the respective male and female components providing rotational positioning for threading without wedging. Such positional requirements may be particularly difficult to obtain in applications where large feed and rotational forces are used to mate corresponding components. For instance, in the automated making of coring rod connections in the drilling industry, the equipment may operate with sufficient forces such that jamming, wedging, or cross-threading is an all too common occurrence.
Furthermore, when joining male and female components that are in an off-center alignment, tail-type connections may also be prone to cross-threading, jamming, and wedging. Accordingly, when the male and female components are fed without rotation, the tail can wedge into a mating thread. Under rotation, the tail may also wedge into a mating thread. Wedging may be reduced, but after a threading opportunity (e.g., mating the tip of the tail in opening adjacent a mating tail), wedging may still occur due to the missed threading opportunity and misalignment. Off-center threads may be configured such that a mid-tail crest on the mail component has equal or corresponding geometry relative to the female thread crest.
As discussed above, threaded connectors having tail-type thread starts can be particularly prone to thread jamming, cross-threading, wedging, joint seizure, and the like. Such difficulties may be particularly prevalent in certain industries, such as in connection with the designs of coring drill rods. The thread start provides a leading end, or first end, of a male or female thread and mates with that of a mating thread to make a rod or other connection. If the tail-type thread starts jam, wedge, cross-thread, and the like, the rods may need to be removed from a drill site, and can require correction that requires a stop in drilling production.
Additionally, drill rods and casings commonly make use of tapered threads and tapered joints such that the diameters at the thread starts are smaller than the diameters at the thread ends. Tapered threads and joints reduce the amount of cross-sectional material available to transfer loads. Tapered threads and joints are also prone to cross-threading difficulties. Since a coring rod may have a tapered thread, the tail at the start of the male thread may be smaller in diameter than that of the start of the female thread. As a result, there may be transitional geometry at the start of each thread to transition from a flush to a full thread profile. Because the thread start and transitional geometry may have sizes differing from that of the female thread, the transitional geometry and thread start may mate abnormally and wedge into each other.
If there is a sufficient taper on the tail, the start of the male thread may have some clearance to the start of the female thread, such as where the mid-tail geometry corresponds to the geometry of the female thread. However, the transitional geometry of the start of the thread may nonetheless interact abnormally with turns of the thread beyond the thread start, typically at subsequent turns of mating thread crests, thereby also resulting in jamming, cross-threading, wedging, and the like. Thus, the presence of a tail generally acts as a wedge with a mating tail, thereby increasing the opportunity and probability of thread jamming.
The limitations of tail-type thread designs are typically brought about by limitations of existing machining lathes. In particular, threads are typically cut by rotational machining lathes which can only gradually apply changes in thread height or depth with rotation of the part. Accordingly, threads are generally formed to include tails having geometry and tails identical or similar to other portions of the thread start. For instance, among other things, traditional lathes are not capable of applying an abrupt vertical or near vertical transition from a flush to full thread profile to rotation of the part during machining. The gradual change is also required to remove sharp, partial feature edges of material created where the slight lead, or helix angle, of the thread meets the material being cut.
Existing thread designs do not necessarily make effective use of available material. As explained previously, use of overall root and thread taper results in loss of cross-sectional area of a component, and the loss of cross-sectional material results in reduced load capacity and fatigue strength for a given component. In another instance, use of a single thread provides for ease of manufacture and ease of make and break. However, the use of a single thread limits the pressure flank bearing surface area, thus, the load efficiency of the component. This practice also limits the material at the thread flank-to-thread root interface, the location of maximum stress and for fatigue failure crack initiation, and the fatigue strength of the component.
Furthermore, existing thread designs using a single thread result in components that are inherently unbalanced when mating components are brought into contact. Without wishing to be bound by theory and/or simulation, when drill string components having a single-start thread are brought into mating contact, the pin thread is placed in tension and the box thread is placed in compression. It follows that, since the load in a threaded joint moves to the first point of mated contact, there is a higher portion of load taken by the portion of mated thread nearest the first point of contact on one side of the joint. This unsymmetrical load response can create a bending load in mated drill string components and can detract from load capacity.
Wear is the erosion or displacement of thread material from its original position on the thread surface due to the relative mechanical actions of mating threads. Existing thread designs can also be configured to create an interference fit on, for example, the major diameter of the mating components. For instance, the male thread crest can be configured to create a radial interference with the female thread root. As the threads are made up, the interference fit may be a significant source of thread wear as it can add greatly to the contact pressure between the threads as they slide relative to one another. Ultimately, interference fits on thread features increase thread wear. Thread wear degrades the thread geometry thus the load capacity or load efficiency of the drill string component.
Thus, drawback with traditional threads can be exacerbated with drilling components. In particular, the joints of the drill string components can require a joint with a high tension load capacity due to the length and weight of many drill strings. Furthermore, the joint will often need to withstand numerous makes and breaks since the same drill string components may be installed and removed from a drill string multiple times during drilling of a borehole. Similarly, the drill string components may be reused multiple times during their life span. Compounding these issues is the fact that many drilling industries, such as exploration drilling, require the use of thin-walled drill string components. The thin-wall construction of such drill string components can restrict the geometry of the threads.
Accordingly, a need exists for improved thread designs and drilling components that reduce wear, jamming and cross threading as well as use available material effectively to increase drilling load capacity and joint reliability.